This invention relates to earth anchors. More particularly, the invention relates to screw type earth anchors. Typically, these earth anchors include an augerlike one-piece helically-shaped blade having a sharpened leading edge attached to a shaft, such as by welding. The shaft is typically provided with means for being rotated either manually or by machine to effect engagement of the anchor into the earth. Anchors representative of this design are those disclosed in U.S. Pat. Nos. 3,016,117; 3,645,055; and 3,710,523. It is well known in the art, that while screw anchors of this design do engage themselves in the earth, and are effective in anchoring a member, they are subject to certain shortcomings, one of which requires the application of high torsional forces to the shaft of the anchor so that the blade will cut through the earth. High installation torsional forces are obviously undesirable in that, among other things, larger, more expensive power drive tools are required, and higher stresses are imposed on the anchor. High installation torsional forces are particularly undesirable in the case of manually installed anchors in that in some situations the forces may be sufficiently high so as to preclude complete or proper installation of the anchor. The blades of these anchors are most often welded to the shaft and the torsional forces impose high stresses on the weld. The stresses are concentrated primarily at the junction of the leading edge of the blade and shaft. These high stress concentrations frequently cause failure of the weld with resultant detachment of the blade from the shaft, or result in an undetected weak anchor that can fail in service.
Another shortcoming common with screw type anchors is the imposition of high loads on the generally unsupported trailing edge of the blade by the attached structure after insertion into the ground. The longitudinal forces exerted on the anchor shaft create a bending moment on the trailing edge portion of the blade that can cause the blade to bend or deflect with resultant loss of holding power.
Heretofore, it has been common to reduce required installation torsional force by curving the leading edge of the blade in a spiral fashion, such as that shown in the above-mentioned patents. The curved leading edge reduces reaction moments over the length of the leading edge which in turn reduces the overall required installation torque.
The earth anchor disclosed in U.S. Pat. No. 3,710,523, in addition to employing a curved leading edge, employs a pair of diametrically opposed radially enlarged portions to the blade which help counteract loss of engagement with the earth as the shaft of the arbor is caused to tilt under the forces exerted by the anchored member. While this anchor design helps counteract the tendency of the anchor to tilt within the soil, the bending moments placed on the trailing edge of each of the blades, due to axial forces on the shaft, has for the most part, not been reduced. It can therefore be appreciated that it would be highly desirable to provide for an earth anchor that lessens the required installation torsional force still further from the present state of the art, and also provides increased holding force when the anchor is installed by reducing the bending moments on the trailing edge of the blade. Further, it would be highly desirable to provide for an earth anchor that reduces the possibility of the blade becoming detached from the shaft during installation by reducing stress concentrations at the interface of the blade and shaft.